The genetic basis of human tumor progression was studied by examining two hematopoietic disorders, chronic myelogenous leukemia (CML), and myelodysplasia (MDS). These diseases are characterized by a preleukemic or premalignant phase which progresses to acute leukemia. Using a sensitive tumorigenicity assay, point mutations in the ras protooncogenes were identified in the earliest preleukemic stage of MDS, but in the late or acute leukemic phase of CML. Furthermore, in MDS, the presence of aberrant ras alleles predicted for subsequent progression to acute leukemia. These data suggest that mutant ras genes are important factors in leukemogenesis, but that these mutant ras alleles have different progression functions depending on the disease context. Underscoring the complexity of the progression process, a novel transforming gene (gene X) was identified in the blood cells from two patients with CML. In light of these data, a study of ras mutations in preleukemic states is proposed using a sensitive method for detecting point mutations in small quantities of DNA: polymerase chain reaction. In this fashion, will extend our initial findings in a large number of patients and will determine the clinical significance of these findings. Blood cells will be separated into lineage specific fractions (i.e. granulocytes, monocytes, and lymphocytes) and the stem cell which sustained the mutation in ras can be identified. Lastly, gene X will be cloned from a secondary nude mouse transformant using cosmid vectors. Once cloned, the critical genetic lesion(s) which converted the normal cellular homologue of gene X into its transforming counterpart can be determined.